The present invention concerns an improved inertia switch having a variable operating threshhold, intended particularly for detecting the imminence of locking of a vehicle wheel and the active periods of which are, at least in part, a function of the conditions of adherence encountered locally by the wheels referred to above.
Inertia switches are known, particularly from French Pat. No. 1194791, which are constituted by a flywheel driven by a member integral with a shaft the speed of rotation of which is in accordance with that of the wheel. Or the occurrence of a sufficient deceleration of the wheel, due to braking, the flywheel, due to inertia, pivots with respect to the driving member and this relative movement permits an electric switch to be closed. This switch is generally constituted by two contacts integral respectively, one with the driving member and the other with the flywheel. Resilient and/or centrifugal means permit the decelerations of the wheel to be detected arising only from the braking action on reaching a threshold of deceleration, fixed or variable, as a function of the speed of the vehicle, below which the switch is not operated. These arrangements are particularly described in French Patent No. 2239008.
With these known switches the operating time, that is to say the time during which the two contacts referred to above are in contact, depends particularly on the conditions of adherence encountered by the wheel.
In fact, when vigorously braking a wheel which is in contact with a surface having a strong adherence for example, the deceleration of the wheel is less brutual than if it had been in contact with a surface of weak adherence. The result is, that at the moment of skidding or the commencement of skidding on a surface having a strong adherence, the interia flywheel of the switch has already slowed down. Its energy, at the moment of contact with the driving member, which is dissipated at the level of a coupling by friction of the driving member pushed by the flywheel on the shaft, is reduced and its speed decreases rapidly from a short contact time. On the other hand under conditions of bad adherence, the deceleration of the wheel is very brutual and takes place without, or practically without, previous slowing down of the speed of the interia flywheel. In this case, the result is considerable energy dissipated by the flywheel; hence, too long a contact time.
The role of these switches is to interrupt the supply to the brakes of the wheel under consideration. As soon as there is closure of the switch, braking force diminishes, consequently the wheel can be turned by contact with the ground. In the case of good adherence, the rotation is rapid and often the driving member regains speed before the flywheel has reached a speed equal to the minimal speed of the wheel. This means that the switch opens at a speed of rotation of the rotating assembly higher than the minimal speed achieved by the wheel when locking is imminent. The latter is also a factor reducing the operating time of the switch. In the case of bad adherence, the turning of the wheel is relatively long which means to say that the driving member regains speed slowly leaving the flywheel time to reduce its speed to a speed approaching the minimal speed of the wheel. Consequently, opening of the switch is produced in this case at a speed of the wheel relatively close to the minimal speed.
Opening of the switch controls the application of the brake source on the wheel. In the case of good adherence, this application does not immediately lead to an imminent locking because the wheel is turning at a sufficiently fast speed. On the other hand, in the case of a bad adherence, this application of braking force acts on an insufficiently rotating wheel and provides the risk of leading to locking of the wheel.
The present invention intends to eliminate this risk while proposing an improved switch in which the advantages of known switches are preserved, particularly in the case of good adherence, and which enables the contact to be maintained closed for a longer time in the case of weak adherence so as to permit the wheel time to rotate more correctly.
To this end, the subject matter of the invention is an improved interia switch, having a variable threshold and operating time intended in particular to detect an imminent locking of a vehicle wheel. The switch comprises a rotary device constituted by an interia flywheel, a member coupled by friction to a driving shaft and capable of rotatably driving the flywheel, electrical contact elements carried respectively by the member and the flywheel and capable of making contact when the flywheel pivots with respect to the member and at least one return member coupled between the member and the flywheel. According to the invention, the above contact element coupled to the flywheel is composed on the one hand of a first part integral with a support coupled by friction to the flywheel and capable of being displaced with respect to the latter, cooperating by a single support with a first part of the contact elements coupled to the above referred to member connected to the driving shaft after a relative rotation of the flywheel with respect to the member at least equal to a first given value and, on the other hand, of a second part integral with the flywheel cooperating through skidding with a second part of the contact elements coupled to the above referred to member after a relative rotation of the flywheel and the above referred to member at least equal to a second given value, the second value being greater than the first value.
In one preferred embodiment of the invention, the support referred to above coupled by friction through the flywheel comprises slides for guiding its displacement with respect to the flywheel, constituted by slots the radius of curvature of which is equal to the difference separating them from the axis of rotation of the rotary device in each of which can slide a pin integral with the flywheel, at least one of these pins being provided at its free end with a screw thread co-operating with a lock nut having a resilient washer ensuring the above mentioned friction between the support and flywheel.
The invention will be better understood in the course of the description given hereafter purely by way of example and non-limiting which will permit isolation of the advantages and the secondary characteristics.